Many different types of radiocommunication systems (i.e. networks) exist. GSM, UMTS, LTE and LTE-advanced are non-limiting examples of such radiocommunication systems.
FIG. 1 is a block diagram showing a radiocommunication system. This may be a network structure of a 3rd generation partnership project (3GPP) long term evolution (LTE)/LTE-advanced (LTE-A). An E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) includes at least one base station (BS) 20 providing a user plane and a control plane towards a user equipment (UE) 10. The UE can be fixed or mobile and can be referred to as another terminology, such as a MS (Mobile Station), a UT (User Terminal), a SS (Subscriber Station), MT (mobile terminal), a wireless device, or the like. The BS 20 may be a fixed station that communicates with the UE 10 and can be referred to as another terminology, such as an e-NB (evolved-NodeB), a BTS (Base Transceiver System), an access point, or the like. There are one or more cells within the coverage of the BS 20. Interfaces for transmitting user data or control data can be used between BSs 20 (in the present document, the term “data” is used as a synonymous for “traffic” and does not imply any limitation as to the nature of such data, which can refer e.g. to user traffic or control traffic i.e. signaling). The BSs 20 are interconnected with each other by means of an X2 interface. The BSs 20 are also connected by means of the S1 interface to the EPC (Evolved Packet Core). They may interface to the aGW (E-UTRAN Access Gateway) via the S1. In the example shown in FIG. 1, the BSs 20 are more specifically connected to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MME/S-GW 30 and the BS 20.
Hereinafter, downlink means communication from the BS 20 to the UE 10, and uplink means communication from the UE 10 to the BS 20. In downlink, a transmitter may be a part of the BS 20 and a receiver may be a part of the UE 10. In uplink, a transmitter may be a part of the UE 20 and a receiver may be a part of the BS 20.
FIG. 2 gives an overview of the E-UTRAN architecture where:                eNB, aGW Control Plane and aGW User Plane boxes depict the logical nodes;        The boxes within the eNB box from RRC to Inter Cell RRM as well as the boxes SAE Bearer Control and MM Entity within the aGW Control Plane box depict the functional entities of the control plane; and        The boxes within the eNB box from PHY to RLC depict the functional entities of the user plane.        
Functions agreed to be hosted by the eNB are: Selection of aGW at attachment; Routing towards aGW at RRC activation; Scheduling and transmission of paging messages; Scheduling and transmission of BCCH information; Dynamic allocation of resources to UEs in both uplink and downlink; The configuration and provision of eNB measurements; Radio Bearer Control; Radio Admission Control; Connection Mobility Control in LTE_ACTIVE state.
Functions agreed to be hosted by the aGW are: Paging origination; LTE_IDLE state management; Ciphering of the user plane; PDCP; SAE Bearer Control; Ciphering and integrity protection of NAS signaling.
FIG. 3 shows the user-plane protocol stack for E-UTRAN.
RLC (Radio Link Control) and MAC (Medium Access Control) sublayers (terminated in eNB on the network side) perform the functions such as Scheduling, ARQ (automatic repeat request) and HARQ (hybrid automatic repeat request).
PDCP (Packet Data Convergence Protocol) sublayer (terminated in aGW on the network side) performs for the user plane functions such as Header Compression, Integrity Protection, Ciphering.
FIG. 4 shows the control-plane protocol stack for E-UTRAN. The following working assumptions apply.
RLC and MAC sublayers (terminated in eNB on the network side) perform the same functions as for the user plane;
RRC (Radio Resource Control) (terminated in eNB on the network side) performs the functions such as: Broadcast; Paging; RRC connection management; RB control; Mobility functions; UE measurement reporting and control.
PDCP sublayer (terminated in aGW on the network side) performs for the control plane the functions such as: Integrity Protection; Ciphering.
NAS (terminated in aGW on the network side) performs among other things: SAE bearer management; Authentication; Idle mode mobility handling; Paging origination in LTE_IDLE; Security control for the signaling between aGW and UE, and for the user plane.
RRC uses the following states:
1. RRC_IDLE:
UE specific DRX configured by NAS; Broadcast of system information; Paging; Cell re-selection mobility; The UE shall have been allocated an id which uniquely identifies the UE in a tracking area; No RRC context stored in the eNB.
2. RRC_CONNECTED:
UE has an E-UTRAN-RRC connection; UE has context in E-UTRAN; E-UTRAN knows the cell which the UE belongs to; Network can transmit and/or receive data to/from UE; Network controlled mobility (handover); Neighbour cell measurements; At RLC/MAC level: UE can transmit and/or receive data to/from network; UE also reports channel quality information and feedback information to eNB.
The network signals UE specific paging DRX (Discontinuous Reception) cycle. In RRC Idle mode, UE monitors a paging at a specific paging occasion of every UE specific paging DRX cycle. The paging occasion is a time interval where a paging is transmitted. UE has its own paging occasion. A paging message is transmitted over all cells belonging to the same tracking area. If UE moves from a tracking area to another tracking area, UE will send a tracking area update message to the network to update its location.
A physical channel transfers signaling and data between UE L1 and eNB L1. As shown in FIG. 5, the physical channel transfers them with a radio resource which consists of one or more sub-carriers in frequency and one more symbols in time. 6 or 7 symbols constitute one sub-frame which is 0.5 ms in length. The particular symbol(s) of the sub-frame, e.g. the first symbol of the sub-frame, can be used for the PDCCH (Physical Downlink Control Channel). PDCCH channel carries L1 signaling.
A transport channel transfers signaling and data between L1 and MAC layers. A physical channel is mapped to a transport channel.
Downlink transport channel types are:
1. Broadcast Channel (BCH) used for transmitting system information
2. Downlink Shared Channel (DL-SCH) characterised by: support for HARQ;
support for dynamic link adaptation by varying the modulation, coding and transmit power; possibility to be broadcast in the entire cell; possibility to use beamforming; support for both dynamic and semi-static resource allocation
3. Paging Channel (PCH) used for paging a UE
4. Multicast Channel (MCH) used for multicast or broadcast service transmission.
Uplink transport channel types are:
1. Uplink Shared Channel (UL-SCH) characterised by: possibility to use beamforming; (likely no impact on specifications); support for dynamic link adaptation by varying the transmit power and potentially modulation and coding; support for HARQ
2. Random Access Channel(s) (RACH) used normally for initial access to a cell.
The MAC sublayer provides data transfer services on logical channels. A set of logical channel types is defined for different kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred.
A general classification of logical channels is into two groups:                Control Channels (for the transfer of control plane data);        Traffic Channels (for the transfer of user plane data).        
Control channels are used for transfer of control plane data only. The control channels offered by MAC are:                Broadcast Control Channel (BCCH)        
A downlink channel for broadcasting system control information                Paging Control Channel (PCCH)        
A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE.                Common Control Channel (CCCH)        
this channel is used by the UEs having no RRC connection with the network.                Multicast Control Channel (MCCH)        
A point-to-multipoint downlink channel used for transmitting MBMS control data from the network to the UE.                Dedicated Control Channel (DCCH)        
A point-to-point bi-directional channel that transmits dedicated control data between a UE and the network. Used by UEs having an RRC connection.
Traffic channels are used for the transfer of user plane data only. The traffic channels offered by MAC are:                Dedicated Traffic Channel (DTCH)        
A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user data. A DTCH can exist in both uplink and downlink.                Multicast Traffic Channel (MTCH)        
A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.
In Uplink, the following connections between logical channels and transport channels exist:                DCCH can be mapped to UL-SCH;        DTCH can be mapped to UL-SCH.        
In Downlink, the following connections between logical channels and transport channels exist:                BCCH can be mapped to BCH;        PCCH can be mapped to PCH;        DCCH can be mapped to DL-SCH;        DTCH can be mapped to DL-SCH;        MCCH can be mapped to MCH;        MTCH can be mapped to MCH;        
Conventionally, only one carrier (e.g. a frequency band) is used at a time with respect to a given UE for transporting data, such as useful data and/or control data.
But for supporting wider transmission bandwidths, it would be better to use carrier aggregation, that is simultaneous support of multiple carriers. Carrier aggregation would thus involve transporting data, such as useful data and/or control data, over a plurality of carriers with respect to a given UE. It would thus enhance the conventional carrier usage and be adapted to the multiple access type of the considered radio communication system.
As far as LTE is concerned, carrier aggregation has been introduced in a recent version thereof, so-called LTE-Advanced, which extends LTE Release 8 (LTE Rel-8). Some aspects of carrier aggregation are disclosed for example in 3GPP TR 36.814 V0.4.1, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects (Release 9) released in February 2009 (see section 5 in particular), as well as in subsequent versions thereof. Other standard documents, which are well known by one skilled in the art, relate to other aspects of carrier aggregation.
Thus LTE-Advanced allows having two or more carriers, so-called component carriers (CCs), aggregated in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation.
In contrast with an LTE Rel-8 terminal, an LTE-Advanced terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.
According to a non-limiting example, a carrier may be defined by a bandwidth and a center frequency. If five carriers are assigned as granularity of carrier unit having a 5 MHz bandwidth, carrier aggregation may lead to a bandwidth of a maximum of 25 MHz.
Contiguous spectrum aggregation and/or non-contiguous spectrum aggregation may take place. The contiguous spectrum aggregation uses contiguous carriers and the non-contiguous spectrum aggregation uses discontiguous carriers. The number of aggregated carriers may be different in uplink and downlink. When the number of downlink carriers and that of uplink carriers are equal, it is called a symmetric aggregation, and when the numbers are different, it is called an asymmetric aggregation.
The size (i.e., the bandwidth) of multiple carriers may vary. For example, when five carriers are used to configure a 70 MHz band, they may be configured as 5 MHz carrier (carrier #0)+20 MHz carrier (carrier #1)+20 MHz carrier (carrier #2)+20 MHz carrier (carrier #3)+5 MHz carrier (carrier #4).
FIG. 6 illustrates an example of a protocol structure for supporting multiple carriers. A common medium access control (MAC) entity 210 manages a physical (PHY) layer 220 which uses a plurality of carriers. A MAC management message transmitted by a particular carrier may be applied to other carriers. The PHY layer 220 may operate e.g. in a TDD (Time Division Duplex) and/or FDD (Frequency Division Duplex) scheme.
There are several physical control channels used in the physical layer 220. A physical downlink control channel (PDCCH) may inform the UE about the resource allocation of paging channel (PCH) and downlink shared channel (DL-SCH), and hybrid automatic repeat request (HARQ) information related to DL-SCH. The PDCCH may carry the uplink scheduling grant which informs the UE about resource allocation of uplink transmission. A physical control format indicator channel (PCFICH) informs the UE about the number of OFDM symbols used for the PDCCHs and is transmitted in every subframe. A physical Hybrid ARQ Indicator Channel (PHICH) carries HARQ ACK/NAK signals in response to uplink transmissions. A physical uplink control channel (PUCCH) carries uplink control data such as HARQ ACK/NAK in response to downlink transmission, scheduling request and channel quality indicator (CQI). A physical uplink shared channel (PUSCH) carries uplink shared channel (UL-SCH).
Each component carrier may have its own control channel, i.e. PDCCH. Alternatively, only some component carriers may have an associated PDCCH, while the other component carriers do not have their own PDCCH.
Component carriers may be divided into a primary component carrier (PCC) and one or several secondary component carriers (SCCs) depending on whether they are activated. A PCC refers to a component carrier that is constantly activated, and an SCC refers to a component carrier that is activated or deactivated according to particular conditions. Activation means that transmission or reception of traffic data is performed or traffic data is ready for its transmission or reception. Deactivation means that transmission or reception of traffic data is not permitted. In the deactivation, measurement is made or minimum information can be transmitted or received. The UE generally uses only a single PCC and possibly one or more SCCs along with the PCC.
A PCC is a component carrier used by a BS (i.e. an eNB in the context of LTE/LTE-A) to exchange traffic and PHY/MAC control signaling (e.g. MAC control messages) with a UE. SCCs carriers are additional component carriers which the UE may use for traffic, only per BS's specific commands and rules received e.g. on the PCC. The PCC may be a fully configured carrier, by which major control data is exchanged between the BS and the UE. In particular, the PCC is configured with PDCCH. The SCC may be a fully configured component carrier or a partially configured component carrier, which is allocated according to a request of the UE or according to an instruction of the BS. The PCC may be used for entering of the UE into a network or for an allocation of the SCC. The primary carrier may be selected from among fully configured component carriers, rather than being fixed to a particular component carrier. A component carrier set as an SCC carrier may be changed to a PCC.
A PCC may further have at least some of the following characteristics:                to be in accordance with the definitions of the PCC introduced in Rel-10 CA;        uplink PCC and downlink PCC may be configured per UE;        uplink PCC may be used for transmission of L1 uplink control data;        downlink PCC cannot be de-activated;        re-establishment may be triggered when the downlink PCC experiences RLF (radio link failure), not when other downlink CC's experience RLF;        SI (system information) reception for the downlink PCC, Rel-8 procedures may apply;        this may not imply anything for the reception of the SI of other configured CC's;        NAS information may be taken from the downlink PCC cell.        
With respect to a given UE and a given eNB or base station, the uplink and downlink PCCs are designated by the common term PCell (i.e. primary serving cell), while the uplink and downlink SCCs are designated by the common term SCell (i.e. secondary serving cell), as well known in the art.
When considering Carrier Aggregation (CA), an SCell activation/deactivation mechanism may be carried out in order to increase the data rate (uplink or/and downlink). In this case, a PCell may carry an explicit activation/deactivation of a configured SCell signalled e.g. by a MAC control element (MAC CE). For more details about MAC CEs, the reader can refer for example to the technical specification 3GPP TS 36.321 V8.8.0 (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification (Release 8).
When an eNB sends an activation/deactivation MAC CE command to a UE for activating/deactivating an SCell, it must then wait for a certain amount of time before it can start or stop scheduling the UE on the newly activated/deactivated SCell. Likewise, the UE cannot immediately start or stop sending and reporting data or information (e.g. SRS, CSI, etc.) with respect to the newly activated/deactivated SCell.
This is partly because the UE needs some time to decode the activation/deactivation command and to take consequential changes in its receiving chain.
Even if it is assumed that the UE always receives activation/deactivation command on the first attempt (without a need for a retransmission), still the activating/de-activating carrier aggregation may have impacts on the UE reception e.g. due to the need for radio frequency (RF) retuning when the UE effective Rx and/or Tx bandwidth is modified. For example, in an intra-band contiguous case, it would be necessary to configure the receiver to have wider reception bandwidth (e.g. change reception bandwidth from 20 MHz to 40 MHz) or in reverse to have narrower bandwidth (e.g. changing reception bandwidth from 40 MHz to 20 MHz). This would require UE to adjust the local oscillator (LO) position and reconfigure the baseband filters.
When the UE is performing RF retuning, it cannot receive scheduling transmissions on any of the activated DL SCCs in the same frequency band, as well as on the corresponding UL CCs. Moreover, the UE is not able to receive the DL HARQ ACK/NACK for a corresponding UL transmission happening just before RF retuning start.
As a result, if the activation/deactivation command is sent by the eNB in order to be received in subframe n by the UE, the eNB and the UE may have to wait until the subframe n+X for the SCell to be ready for operation. This situation is illustrated in FIG. 7 for an exemplary SCell activation case.